Communication devices such as wireless devices may also be known as e.g. user equipments (UEs), mobile terminals, wireless terminals and/or mobile stations. A wireless device is enabled to communicate wirelessly in a cellular communication network, wireless communication system, or radio communication system, sometimes also referred to as a cellular radio system, cellular network or cellular communication system. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communication network. The wireless device may further be referred to as a mobile telephone, cellular telephone, laptop, Personal Digital Assistant (PDA), tablet computer, just to mention some further examples. The wireless device may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile device, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device or a server.
The cellular communication network covers a geographical area which is divided into cell areas, wherein each cell area is served by at least one base station, or Base Station (BS), e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is typically identified by one or more cell identities. The base station at a base station site provides radio coverage for one or more cells. A cell is thus associated with a geographical area where radio coverage for that cell is provided by the base station at the base station site. Cells may overlap so that several cells cover the same geographical area. By the base station providing or serving a cell is meant that the base station provides radio coverage such that one or more wireless devices located in the geographical area where the radio coverage is provided may be served by the base station in said cell. When a wireless device is said to be served in or by a cell this implies that the wireless device is served by the base station providing radio coverage for the cell. One base station may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the wireless device within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunication System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communication (originally: Groupe Special Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or eNBs, may be directly connected to other base stations and may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which may be referred to as 3rd generation or 3G, and which evolved from the GSM, and provides improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for wireless devices. High Speed Packet Access (HSPA) is an amalgamation of two mobile telephony protocols, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), defined by 3GPP, that extends and improves the performance of existing 3rd generation mobile telecommunication networks utilizing the WCDMA. Such networks may be named WCDMA/HSPA.
The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies, for example into evolved UTRAN (E-UTRAN) used in LTE.
The expression downlink (DL) is used for the transmission path from the base station to the wireless device. The expression uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless device to the base station.
Handover refers to the action of handing over a wireless device being served in a serving cell to another, target cell to instead being served in there, e.g. in order to enable seamless service provision during mobility, i.e. when the wireless device moves. Decisions whether to perform handover or not are based on handover parameters and measurements performed by the wireless device on a serving, source cell and on neighbouring, potential target cells for the handover. The handover parameters for example determine conditions or criteria to be fulfilled by the measurements in order for handover to be carried out. If the handover parameters are set in such a way that the wireless device doesn't report handover measurements in time, the wireless device might lose the connection with the source cell before a handover is initiated. The wireless device can also lose a connection if the wireless device enters a so called coverage hole where there e.g. is not sufficient radio coverage. As described in 3GPP TS 36.331, version 12.4.1, “Radio Resource Control”, chapter 5.3.11.1, when the wireless device receives a certain number, named N310, of consecutive “out of sync” indications from a lower layer, it assumes a physical layer problem is ensuing, and a timer, named T310, is started. If the wireless device doesn't receive a certain number, named N311, of consecutive “in sync” indications from the lower layer before the T310 expires, Radio Link Failure (RLF) is detected.
Radio link failure is a functionality that should “protect” the wireless device communication when network based mobility does not work properly, e.g. so that a wireless device when losing sufficient radio coverage from a currently serving cell, but before receiving a command to go to a different cell, may select a different cell, using the same or different carrier frequency and/or RAT.
When an RLF, or HandOver Failure (HOF), is detected by the wireless device, the wireless device starts a timer, named T311, and tries to re-establish the connection to the best available cell. See e.g. 3GPP TS 36.331, version 12.4.1, “Radio Resource Control”, chapter 5.3.7.2.
From the above observations regarding the existing RLF occurrence and recovery procedure, one can say that the major disadvantage of the RLF is that there will be a service discontinuity when the wireless device undergoes RLF. This interruption will cause performance degradation. An example of this is given in the following with reference to FIG. 1. A wireless device is assumed to be moving in an LTE wireless communication network and its Reference Signal Received Power (RSRP) as a function of time is shown in FIG. 1. The wireless device calculates thresholds, named Th1 and Th2 in FIG. 1, for declaring out-of-sync (lower line, Th1) and in-sync (upper line, Th2) based on received quality regarding Cell specific Reference Symbols (CRS) where the thresholds are set based on expected block-error rate for specific messages on a Physical Downlink Control Channel (PDCCH), see e.g. 3GPP TS 36.133, version 12.5.0, section 7.6. Below this lower value the wireless device starts to count the number of times it observes consecutive out-of-sync measurements. If this count exceeds the pre-configured number N310 then the wireless device will start the RLF timer T310. In the example of FIG. 1, the wireless device starts the T310 timer at time T1. If the wireless device fails to find sync within the expiry of the pre-defined duration of the T310 timer, then the wireless device will declare RLF. In the example of FIG. 1, the wireless device declares RLF at T2.